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Transistor Amplifiers
Biasing
Biasing for the Active Region
• In order for a transistor amplifier to work the transistor
must be in the active region.
• One option is to bias the transistor by a using a number of
power supplies.
VEE
VCC
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Voltage-Divider Biasing• The most common method of biasing a transistor is to use
a single supply and a voltage divider circuit.
vout
R 24.7 k Ω
2N3904
R 110 k Ω
+VCC
15 V
R E2.7 k Ω
R C
3.9 k Ω
vin
Voltage-Divider Biasing
• The resistors R 1 and R 2 form a
simple voltage divider.
• The DC emitter voltage can be
found from VB.
vout
R 2
4.7 k Ω
2N3904
R 1
10 k Ω
+VCC
15 V
R E2.7 k Ω
R C
3.9 k Ω
vin
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Voltage-Divider Biasing• Likewise the emitter current
can be found from
• The collector current and DC
collector voltage are
.
vout
R 24.7 k Ω
2N3904
R 1
10 k Ω
+VCC
15 V
R E2.7 k Ω
R C
3.9 k Ω
vin
Q-Point
• We still need to determinethe optimal values for theDC biasing in order tochoose resistors, etc.
• This bias point is called thequiescent or Q-point as itgives the values of thevoltages when no inputsignal is applied.
• To determine the Q-point
we need to look at therange of values for whichthe transistor is in theactive region.
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Load Line• At saturation the resistance
offered by the transistor iseffectively zero so thecurrent is a maximumdetermined by VCC and theresistors R E and R C.
• When the transistor is incutoff no current flows soVCE = VCC.
• If we connect these two points with a straight line
we get all possible valuesfor IC and VCE for a givenamplifier.
Q-point
• To determine the q-point weoverlay the load line on thecollector curves for thetransistor.
• The Q-point is where theload line intersects theappropriate collector curve.
• For example if the amplifier
is operated at IB = 20 µA theQ-point is as shown on thegraph.
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Midpoint biased• When the Q-point is chosen so that VCE is half of VCC and
IC is half of IC(sat) the amplifier is said to be midpoint
biased.
Optimum Biasing
• Midpoint biasing allows
optimum ac operation of the
amplifier. When an ac
signal is applied to the base
of the transistor, IC and VCE
both vary about the Q-point.
With the Q-point center the
values can make the
maximum deviations from
the Q-point either above or
below.
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Transistor Amplifiers
Gain and Impedance
Gains
• AC Gain is the ratio between the ac output and ac input
signal.
• Voltage:
• Current:
• Power:
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Decibels• Gains are sometimes expressed in terms of
decibels.
• dB Power Gain:
• dB Voltage Gain
pdB p A A log10)( =
vdBvA A log20)( =
Basic Amplifier Model
• The basic amplifier is characterized by its
gains, input impedance and output
impedance.
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The Ideal Amplifier • The ideal amplifier has infinite input
impedance (R in = ∞), zero output impedance
(R out = 0) and infinite gain (if desired).
Amplifier Input Impedance (Zin)
• If we assume that the input impedance of
our amplifier is purely resistive then the
signal voltage at input (vin) is
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Output Impedance (Zout)• If we assume that the output impedance of
our amplifier is purely resistive then the
signal voltage at output (vout) is
Effects of Input and Output
Impedance
• vs = 15 mV, R s= 20Ω, R L =1.2k Ω
• Avo = 340, Zin = 980Ω, Zout = 250Ω
• vout = 4.14 V, Av(eff) =276
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Effects of Input and Output
Impedance• vs = 15 mV, R s= 20Ω, R L =1.2k Ω
• Avo = 340, Zin = 8k Ω, Zout = 20Ω
• vout = 5.0 V, Av(eff) = 333
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